The notion of a "blue wave" refers to a theorized Democratic sweep during the upcoming midterm elections, specifically the idea that the party could win back control of both the House of Representatives and Senate.

Democrats have enjoyed an overwhelming enthusiasm advantage for much of the Trump era, leading some pollsters and pundits to predict heightened Democratic voter turnout that could flip purple and even some red districts blue.

They hope an explosion of early voting across states like Florida, Texas and Nevada is further proof of their enthusiasm gap. Public and private polling, however, suggests Republican voters are getting more excited as Nov. 6 approaches.

In the House race, the odds are still in the Democrats' favor. As of the final forecast on Tuesday, Nate Silver of FiveThirtyEight projects a 6 in 7 chance that the Democrats will win control of the House.

It's a different story in the Senate race, though. Silver currently projects a 1 in 5 chance that the Democrats will win control of that body and a 4 in 5 chance that Republicans will maintain control. Thirty-five seats are up for grabs, but just a handful are toss-ups, according to Silver's frequently evolving calculations.

There are signs that the Democrats' position in the expanding House battlefield may actually be improving. Yet Republican candidates locked in tight races from New York to Nevada find themselves in stronger-than-expected positions because of a bump in President Donald Trump's popularity, the aftermath of a divisive Supreme Court fight and the sudden focus on a caravan of Latin American immigrants seeking asylum at the U.S. border.

"It's still much closer than people think, with a surprise or two in the wings," New York Sen. Chuck Schumer, the top Senate Democrat, told The Associated Press.

The midterm elections will decide whether Republicans maintain control of Congress for the final two years of Trump's first term. Even if Democrats lose the Senate and win the House, they could block much of Trump's agenda and use subpoena power to investigate his many scandals. Some in the party's far-left wing have also vowed to impeach the president, while others promise to roll back the Republican tax overhaul and expand health care coverage for all Americans.